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Heat AC one vent blowing cold

Hello everyone. I hope someone can lead me on the right direction.

I had a new AC 4 ton put in my house over the summer. Now in the winter I turned the heat on (live in South Florida) I have a 10kw heat element. The house seems to warm up slowly, but there is one room that has a vent that feels like no heat is coming out. It feels like regular air as if the fan is on only.

Is very strange, I had the tech that installed it look at it, but he said it was an insulation problem on the attic. I know that room has barely any insulation on top, but shouldn't the vent still blow hot air.

I believe the vent is a 6" vent to the room. From past experience living in other houses. Every time the heater was turned on, you can feel the air coming out of the vents nice and toasty. On this AC unit the vents don't feel that way. the other ones feel warm so eventually it does heat the house, but the problem is the one for the baby room.

Now I remember he did put the laser thing on it, and it was coming out 88 degrees from that room, 96 from another, and 104 on the master room.

I was thinking of having him change the flex duct and routed some other way. (he suggested that), but Im concern about it. I mean very strange that only one vent blows regular air (colder air) compared to the rest.

There are couple of possibilities I can think of that would make the air come out considerably colder in one vent than others.

1 - Its a longer run so the air is cooled down more on its way to the vent.
2 - There could be a leak in the duct
3 - The take-off can be too close to the air handler & the heat from the strips are by-passing the duct. If this is the case there won't be much difference in temperature in cooling from the other vents but there will be in heating.